1. Field of the Invention
The present invention relates to a flyback converter, and more specifically to a flyback converter with no need for the auxiliary winding in the flyback transformer, leading to simplification of transformer structure and reduction in transformer cost.
2. The Prior Arts
Various DC-powered devices, such as integrated circuits, DC motors, or liquid crystal displays without a proper DC voltage source, would need a suitable voltage converter, such as a flyback converter, a forward converter, or an LLC converter for DC-to-DC voltage conversion, to convert a rectified AC voltage source to a specified DC voltage level to power them.
In prior arts, a flyback converter normally has its output voltage regulated either by Secondary-Side Regulation (SSR) or by Primary-Side Regulation (PSR). SSR regulates the output voltage by means of optical coupling from the secondary side, requiring an optocoupler-based feedback control circuit in the secondary side while having the advantage of a tighter output voltage regulation. PSR regulates the output voltage by means of magnetic coupling to the primary side, requiring no optocoupler-based feedback control circuit in the secondary side while having the disadvantage of a looser output voltage regulation.
Also, a traditional flyback converter generally has its primary switch driven by a Low-Side Drive (LSD), which drives a primary switch placed at the low side of a primary winding, requiring an auxiliary winding in the primary side because the primary winding can't fill in for the auxiliary winding to power the primary IC controller with a continuous and steady working voltage after startup.
For the purpose of eliminating the auxiliary winding from a flyback transformer, the present invention discloses a High-Side Drive (HSD), which drives a primary switch placed at the high side of a primary winding, requiring no auxiliary winding in the primary side because the primary winding can fill in for the auxiliary winding to power the primary IC controller with a continuous and steady working voltage after startup.
Please take a look at FIG. 1 illustrating a traditional LSD-SSR flyback converter with an auxiliary winding NA. The traditional LSD-SSR flyback converter comprises an LSD flyback converter placed across the primary and secondary side for voltage conversion and power delivery, an optocoupler 92-based feedback network placed in the secondary side for SSR, and a Pulse Width Modulation (PWM) controller 90 placed in the primary side for controlling and driving the LSD-SSR flyback converter. Further, the transformer 40, which comprises a primary winding NP, a secondary winding NS, and an auxiliary winding NA needing to be present for continuously and steadily powering the primary PWM controller 90 because the primary winding NP can't lend itself to continuously and steadily powering the primary PWM controller 90 in the absence of the auxiliary winding NA.
The PWM controller 90 starts switching the switch transistor 80 on and off when the unregulated input voltage source VIN charges the VCC capacitor 34 up to the startup level through the startup resistor 22 after power-on. The LSD-SSR flyback converter gets into its steady-state operation after the auxiliary winding NA takes over the continuous and steady working voltage supply by replenishing the VCC capacitor 34 with an induced voltage (≈VO/NS*NA) through the diode rectifier 14 as long as the working voltage stays above the Undervoltage Lockout (UVLO) level.
FIG. 1 shows an LSD (Low-Side Drive), where the switch transistor 80, which can be but won't be limited to a Metal-Oxide Semiconductor Field Effect Transistor (MOSFET)/a Bipolar Junction Transistor (BJT), in series with a current-sensing resistor 25 is placed at the low side of the primary winding NP and the IC GND is connected and referenced to the source/emitter through the current-sensing resistor 25 to drive the switch transistor 80 with source/emitter-referenced PWM signal.
FIG. 1 also shows a Secondary-Side Regulation (SSR), where the output voltage VO gets regulated by an optocoupler-based feedback loop through an optical coupling signal from the secondary side. It is worth mentioning that the diode rectifier 19 can be placed either at the secondary high side or at the secondary low side with no influence on the output voltage sense through the optocoupler-based feedback loop.
Now, please take another look at FIG. 2, which shows an LSD-PSR flyback converter. Identical/similar to that in FIG. 1, the LSD in FIG. 2 wouldn't be restated herein. Only PSR as well as continuous and steady working voltage supply, which rely heavily on the auxiliary winding NA, need to be detailed hereafter. With reference to the PWM controller in FIG. 2, the FB pin would be internally clamped at a slightly negative/positive potential (−0.3V/0.15V typical), protecting the FB pin against an excessively negative voltage, when the primary power BJT switches on to store energy and the auxiliary winding NA induces a negative voltage
      -                  N        A                    N        P              ⁢            V      I        .  The FB pin would sense a scaled-down reflected output voltage
                    N        A                    N        S              ⁢          V      O        ×                  R        2                              R          1                +                  R          2                      ,which is used to execute PSR, when the secondary power diode turns on to release energy and the auxiliary winding NA induces a positive voltage
                    N        A                    N        S              ⁢          V      O        ,which is used to replenish the VCC capacitor. It's crystal clear that the auxiliary winding NA here wears two hats: PSR as well as continuous and steady working voltage supply.
Having their primary switch low-side driven, the traditional flyback converters, be they secondary-side regulated (FIG. 1) or primary-side regulated (FIG. 2), all need an auxiliary winding, which can be further left out using a High-Side Drive (HSD), as will be elaborated in the following paragraphs.